1. Field of the Invention
The invention relates to a device for screening off lines of electric flux in an installation for electrolytic processing of substantially plate-shaped material in the form of one or more workpieces which is held underneath a goods carrier and immersed in a bath when the goods carrier is lowered. A screen is provided which is adjustable in the vertical direction by hold-down means and screens off the above-mentioned lines of electric flux at the lower region of the material to be processed in the course of operation, the hold-down means being coupled with the movement of the goods carrier. Its preferred use is for electroplating printed circuit boards. The invention could also be used for processing other, similar workpieces and in other installations for electrolytic processing, e.g. for carrying out electrophoresis to apply thin, photo-sensitive films on printed circuit boards.
2. Discussion of the Prior Art
A device constructed for screening off lines of electric flux in an electroplating installation is disclosed in the subsequently published DE-OS 40 05 209 which describes, among other things, a construction with a lower pair of screens which arc constructed as floating bodies and arc situated on both sides of the lower region of a plate-shaped material to be electroplated. They are brought into the desired lower screening off position by hold-down means. Those hold-down means are fastened to frame rods, between which the plate shaped material to be electroplated is supported, and are moved down along with the downward movement of the goods carrier. However, the aforementioned screens which are constructed as floating bodies can only be moved up out of their lower position by a relatively short distance, since they are prevented from a continued upward movement by a stop. It is also possible to raise these lower screens in a corresponding manner by means of a pull cable. The lower screens mentioned above are also guided at the bath vessel so as to be adjustable in the vertical direction. This is also true of the upper screens which are likewise provided. DE-OS 40 05 209 mentions that the invention can be used when the workpieces to be electroplated are suspended at the goods carrier by clamps or the like or also in arrangements in which the workpieces to be processed are held between frame rods of carrying racks which are attached in turn to the goods carrier. However, the aforementioned construction of two bold-down means which press down the lower pair of screens is disclosed only in the form of two hold-down parts at frame rods of a carrying rack. In the subject matter of DE-OS 40 05 209, the force for moving the screens down is applied by the hold-down means, i.e. the plate-shaped material to be processed is not loaded by this force. However, a disadvantage consists in the relatively costly, bulky construction of the two screens and particularly in the requirement that the screens be guided in the bath vessel in the vertical direction by their ends.
Another device for screening off lines of electric flux in an electroplating installation is known from JP 55-145 199 A in Patent Abstracts of Japan, Sect. C, Vol. 5 (1981) No. 16 (C-41). In this case, a screen is provided in a shielding and positioning frame for such printed circuit boards in immersion electroplating installations. The screen is constructed like the ridge of a roof and is guided by lateral tabs into vertically extending slots of a holder. The "ridge" of this "roof" faces downward. In its initial position, this screen is situated on the surface or upper region of the bath. The printed circuit board to be processed is inserted from above into the interior of the "roof" by its lower edge and carries the screen down with it during the immersion process. This screen prevents so-called scorching, i.e. unwanted additional metallization deposits in the lower edge region of the aforementioned printed circuit board. This arrangement of the prior art has a number of disadvantages. The screen must be pressed down by way of the printed circuit board to be processed. But such printed circuit boards are often very thin so that they cannot transmit the forces required for pressing down without sustaining damage. In such cases, the printed circuit boards buckle outward under the influence of the pressing pressure, which can damage these sensitive boards and lead to deposits of irregular thickness on their convex and concave sides. An arrangement corresponding to the aforementioned screening off device is known from DE-PS 37 26 571. A similar arrangement with lateral guides of the screen at the bath vessel is known from U.S. Pat. No. 4,879,007.